A time measurement apparatus for accurately measuring the time interval between signals has become necessary as a result of the increase in the speed of digital communications in recent years. The signal interval is generally found by counting the clocks generated between signal inputs and adding the clock period to the number of counts. However, this measurement method cannot measure time intervals that are shorter than the clock period; therefore, although clock signals of a short period are necessary for obtaining good measurement accuracy, there are limits to the operating speed of the counter and this in turn limits measurement accuracy. Consequently, means for measuring times shorter than the clock period using a time-voltage converter and analog-digital converter (ADC) have been added to the conventional measurement method using a counter in order to make very accurate measurements possible without raising the clock frequency. The structure of a typical measurement apparatus of this type is shown in FIG. 2.
The time measurement apparatus in FIG. 2 is a measurement apparatus for measuring the time interval from when a signal edge is input to the start input to when a signal edge is input to the stop input. It comprises a ramp generator 100 for generating ramp signals, which is connected to the start input; a clock generator 101; a sample hold circuit (S/H circuit) 102 for holding ramp signal voltage during one clock period, which is connected to ramp generator 100 and clock generator 101; an analog-digital converter (ADC) 103 for converting S/H signals to digital signals, which is connected to S/H circuit 102; a ramp generator 200 to which the “stop signal input” has been connected; a sample hold circuit (S/H circuit) 202 for holding ramp signal voltage for one clock period, which is connected to ramp generator 200 and clock generator 101; an analog-digital converter (ADC) 203 for converting S/H signals to digital signals, which is connected to S/H circuit 202; a counter 104 for counting clock signals from event detection signal 1 to event detection signal 2, this counter being connected to clock generator 101 and ramp generators 100 and 200; and a processor 105 for calculating the time from start signal input to stop signal input, this processor being connected to clock generator 101, ADCs 103 and 203, and counter 104.
The operation of the time measurement apparatus in FIG. 2 will be described while referring to FIG. 3. Ramp generator 100 outputs a pre-determined voltage under normal conditions, but when a measurement signal is input to the “start input”, ramp signals that increase linearly from a pre-determined voltage are output based on the rising edge of the measurement signals. Under normal conditions, S/H circuit 102, which has input the ramp signals, continuously outputs a certain voltage without ramp operation, but when the rising edge of first clock signal (CLK) after “start input” is input, the ramp output is held for one CLK period. During this holding time, processor 105 finds a time interval (T1) from when measurement signals are input to “start input” up to the CLK input that immediately follows based on the sample data obtained when ADC 103 has converted the input voltage to digital values and the potential difference from the voltage output by the ramp generator under normal conditions. At the same time, a time interval (T2) from when measurement signals are input to “stop input” up to “CLK signal input” that immediately follows can be found. S/H circuit 202 returns to the voltage under ordinary conditions once the holding period is over.
On the other hand, when measurement signals are input to “start input”, ramp generator 100 outputs event detection signals to counter 104. These signals are generated as signals that are delayed by one period from the rise of the next CLK of the start signal. These event detection signals are handled as reset signals. The count of counter 104 is set at 0 by this reset signal and counts up from the next pulse input. Consequently, the number of clocks (N) generated from the “start signal input” to the “stop signal input” can be obtained by referring to the count value when event detection signals are generated from ramp signal generator 200 by the “stop signal input”.
Processor 105 calculates the time interval (T) from the “start signal input” to the “stop signal input” based on these measurement results. Specifically, when the CLK period is TC, T=N×TC+T1−T2.
Thus, it becomes possible to measure a time interval that is shorter than the time until the ramp generator returns to ordinary conditions by using two sets comprised of a combination of a ramp generator, an S/H circuit, and an ADC, with one set being employed for the signal from the start input and the other set being employed for the signal from the stop input.
By means of the above-mentioned measurement apparatus in FIG. 2, the measurement of a time interval that is much more accurate than the clock signal is possible theoretically. However, there is a problem in that sufficient measurement accuracy is not actually realized because a high-speed time-voltage converter or an ADC having good linear conversion properties does not exist. Therefore, technology exists where measurement accuracy is improved by pre-inputting calibration signals into the measurement apparatus and calibrating by the values found from these measurement results, as disclosed in JP (Kokai) 9[1997]-171,088. By means of this calibration method, calibration signals having different periods are generated randomly as measurement signals, the frequency distribution (histogram of sample count) of the measured calibration signals versus time is charted, and the measurement is calibrated by the value found from this frequency distribution. If the number of samples is sufficient, the frequency distribution should be approximately uniform; therefore, if the calibration value is determined such that the frequency distribution is equalized, a value that is approximately the true value is obtained.
However, it takes a very long time to determine the calibration value even when there are enough samples to improve accuracy. Moreover, there is no method for generating completely random numbers. Therefore, there is a problem in that measurement accuracy cannot be guaranteed.